Reflective optical elements for the EUV and soft X-ray wavelength range may, for example, be multilayers. Multilayers are assembled from periodical repetitions, whereby in the simplest case one period consists of two layers. One layer material should preferably show a high real part of the refractive index, while the other layer material should show a low real part of the refractive index. The layer with the high real part of the refractive index is also called spacer; the layer with the small real part of the refractive index is also called absorber. The thickness of the period as well as the thickness of the individual layers is chosen depending upon the operating wavelength, so that the reflectivity is maximized at this operating wavelength.
Multilayers are used, for example, in EUV lithographic devices for the production of semiconductor devices. During operation, the multilayers are exposed to both a radiation of up to 20 mW/mm2 EUV photon density or more and a relatively high portion of residual gas of water, oxygen, and hydrocarbons. The residual gases are broken down to reactive cleavage products, which cause degradation and contamination of the multilayer surface, by irradiation with EUV photons or secondary electrons or by the influence of external electrical fields. These contaminations cause imaging defects. In the worst case, an image is completely prevented by them. During the operation of the multilayer, cleaning cycles have therefore to be arranged. These significantly increase the operational costs.
In U.S. Pat. No. 6,228,512 B1 it is proposed to allow for a protective coating of SiO2, Zr2O, or ZnO that does not react with water, on a MoRu/Be-multilayer. ZnO in particular is recommended, since during the deposition of Zn an only 0.5 to 0.6 nm thick intrinsic ZnO-layer is formed, that sufficiently protects the multilayer against oxidation without—due to its low thickness—significantly deteriorating the reflectivity.
In U.S. Pat. No. 5,958,605 a special protective coating system for EUV-multilayer is proposed, in which a lower layer of silicon or beryllium is allowed for, that is directly deposited on the multilayer, and at least one upper layer, that on its part is deposited on the lower layer, whereby this upper layer comprises a material that is resistant against oxidation and corrosion and also protects the underlying layers against oxidation.
The protective coatings according to U.S. Pat. Nos. 5,958,605 and 6,228,512 B1 give in fact protection against aging through the influence of oxygen. Contamination by carbon-containing substances does, however, still occur. These cause uncontrolled reflectivity losses and wave front changes. Cleaning cycles must therefore still be performed. The cleaning cycles not only increase the hold-up times, but also harbor the risk of the deterioration of the layer homogeneity of the multilayer or the risk of increasing the surface roughness. This also causes reflectivity losses.
Against this background it is the object of the invention to provide a reflective optical element for the EUV and soft X-ray wavelength range or a corresponding protective coating system, so that the lifetime is increased and the hold-up times are decreased.